The present invention relates to power line communication systems and, particularly, to power line communication systems communicating high frequency signals over existing residential power wiring.
Normally, cable television and regularly broadcasted television signals are routed through cable or flat wire, respectively, to various televisions located at various locations within a residential or commercial facility. When adding a new television, additional cable or flat wire is routed to accommodate the new televisions.
Recently, there have been attempts to communicate over preexisting power distribution networks comprising power, neutral and ground lines routed throughout the residential or commercial facility thereby providing power and signal sources at variously distributed sockets, for example, standard duplex three-prong connectors.
Certain systems have been used to communicate between transmitters and receivers connected to the power distribution networks providing power and communicating signals therebetween.
Some of the earlier versions of these systems utilize conductors of power distribution networks which can be readily tailored to accommodate successful communication. Systems communicating stereophonic signals, closed circuit television signals, digital signals, image data signals, and radio signals over the primary power distribution networks have been proposed and developed. The most significant problem encountered by these systems is noise.
The previous systems which communicate over residential branch circuits have traditionally utilized the power and neutral conductors as dual communication lines or have used the neutral and ground conductors as dual communication lines. These dual communication lines either generate noise or are receptive to noise by connected loads. Either type of noise poses a significant obstacle to successful communication.
The most common sources of noise in residential power distribution networks are various current switching devices. Typically, such noise is basically transient in nature but may be generated repetitively as in the case of motors. Noise can appear as transient voltages between any two pair of lines of the power distribution network as well as appear as current flowing through the lines of the distribution network. Transient voltage magnitudes can typically range up to 300 volts with frequency components ranging from sixty hertz to hundreds of megahertz.
High frequency noise components pose a particular problem to the transmission of high frequency signals. Heretofore, systems which have been developed for successful power line communications have had to limit their frequency range to lowband regions below hundreds of kilohertz. Because of the use of dual communication lines by the heretofore prior art systems, the noise components with respect to the same have been substantial and have prohibited operation in the high frequency range i.e. hundreds of mega hertz range. The higher the frequency, the smaller the signal that can be transmitted. Hence, the signal to noise ratio is much lower when transmitting at higher frequencies thereby limiting the frequency range of high frequency transmissions. The noise component associated with dual line transmission is of such a great magnitude that the heretofore prior art systems have not been able to successfully communicate in the hundreds of mega hertz range.
As the current loads vary in response to a number of active varying loads on a power distribution network, the amount of current and the voltage level thereon correspondingly varies. It is known that the actual impedance of the dual transmission lines vary as the current and voltage thereon vary. Hence, the impedance into a given point along the power distribution lines varies with corresponding varying loads. It is also well known that impedance matching between the output of dual communication lines and the input of a receiving circuit provides maximum energy transfer and minimizes signal attenuation at the receiving circuit.
Heretofore, power line communications systems have incorporated fixed impedance matching as a means to match the distribution line impedance to a transmitter or receiver input impedance thereby transmitting and receiving at the most effective level. That is to say, when the transmitter or receiver input impedance equals that of the impedance into the distribution line, a maximum energy transfer occurs so as to transmit and receive with the maximum signal possible.
Typically, such prior art systems have measured the line impedance at the time of installation and have set the input impedance of receivers or transmitters to that impedance level thereby matching the distribution line impedance solely at the time of installation. However, because the distribution line varies its impedance from time-to-time in response to varying loads, the match of impedance between the distribution line and the input of the transmitter or receiver changes, resulting in an impedance mismatch and attenuation of the signal from time-to-time. Examples of such prior art systems are numerous.
In Stradley U.S. Pat. No. 3,369,078, stereophonic signals are transmitted with a 1.7 mHz carrier having audio modulation. The transmission occurs on two power lines of an existing power distribution network. A variable inductor providing fixed impedance matching is used to tune the circuits to the carrier frequency.
In Chou et al. U.S. Pat. No. 4,054,910, closed circuit television signals are transmitted using differential input and output wires. DC matching is fixed at time of installation by the use of a potentiometer. The frequency transmission occurs between 5 hertz and 5 mHz which transmission is limited by the frequency range of the amplifier. The circuit therein described requires repeater circuits to compensate for line loss, that is, signal attenuation over the power distribution network.
In Howell U.S. Pat. No. 4,408,186, communication occurs at 160 kilohertz using dual line transmission upon the neutral and ground lines. Again, matched impedance is fixed by inductors.
In Kabat et al. U.S. Pat. No. 4,429,299, digital signals are transmitted between 50 kilohertz and 2 mHz. The system therein disclosed includes transceivers for two way data communications. The neutral and ground lines are used for dual line transmission.
In Moriguchi et al. U.S. Pat. No. 4,451,853, image data transmission of scanned documents is transmitted at about 400 kilohertz and Dual line coupling is required.
In Hamlin et al. U.S. Pat. No. 4,507,646, radio communication having a 20 kilohertz to 500 kilohertz carrier with audio modulation is transmitted over existing power lines. Again, this communication is transmitted over dual power lines. Again, impedance matching is fixed at time of installation by selecting a particular component value and does not correct for variable impedance that occurs in the lines from time to time during use.
A review of these prior issued patents suggest that the current teachings require dual line transmission. The applicant's of the subject application have recognized the limited communicative frequency range due to the low signal to noise ratio that occurs when transmitting at higher frequencies over dual transmission lines. This frequency limitation and fixed impedance mismatching attenuating transmitted signals are solved or reduced by the subject invention.